Drug delivery systems for cancer therapeutics have now been used by millions of patients and have resulted in the creation of new therapies and the improvement of existing ones. Cancer drugs can cause enormous toxicity at the systemic concentrations needed to achieve antitumor activity; therefore, their local delivery creates the possibility of improving both their safety and efficacy.
Consequently, locally implanted polymeric devices have gathered clinical interest during the last decades. One example is provided by the carmustine-loaded wafers for the treatment of gliomas. Such formulations consist in solid discs of the drug carmustine (BCNU) loaded in the polyanhidride polymer poly-[bis(p-carboxyphenoxy)propane sebacic acid. BCNU-loaded wafers are approved for the treatment of brain tumors (gliomas) after surgery (Attenello et al., Use of Gliadel (BCNU) Wafer in the Surgical Treatment of Malignant Glioma: A 10-Year Institutional Experience. Ann. Surg. Oncol., 2008 15(10):2887-93).
Polymeric nanofibers have been proposed as malleable platforms for tissue engineering and as carriers to deliver therapeutic agents locally at specific sites of application. Such nanofiber-based systems combine several important aspects, such as large surface area and high porosity which facilitate permeability to water and diffusion of active agents incorporated into the nanofibers. Three distinct techniques have been proven successful in routinely creating nanofibrous structures: (i) self-assembly, (ii) phase separation and (iii) electrospinning.
Self-assembly, as such used to synthesize nanofibers from peptide amphiphiles is attractive because of the mild condition of fabrication and the small size attainable. Patent application WO 2008/067145 provides a method for nanofiber formation from self-assembling peptides. However, this technique is amenable only to a limited repertoire of polymers and difficult to process into a macroscopic structure. It is also challenging to obtain a sustained release kinetics from these small fibers.
The phase separation technique requires gelation of the polymer and extraction of solvent and suffers from a lack of control over fiber arrangement. The required solvent extraction step would also prematurely leach out any drugs entrapped in the fibers. Additionally, only a few polymers are appropriate for this method and it is strictly a laboratory scale technique (Liu et al., The nanofibrous architecture of poly(L-lactic acid)-based functional copolymers, Biomaterials 31 (2010) 259-269).
The electrospinning technique improves the aforementioned methods to obtain nanofibers because it facilitates the scaling-up of the technique and avoids the solvent extraction step. Electrospun nanofibers for biomedical applications have attracted a great deal of attention in the past several years. For example, electrospun nanofibers have been used in tissue engineering, immobilized enzymes and catalyst, wound dressing and artificial blood vessels. They have also been used as barriers for the prevention of post-operative induced adhesion and vehicles for controlled drug delivery systems.
Both mono-axial and co-axial electrospun nanofibers have been reported to incorporate and release antibiotics, drugs and proteins in a sustained manner. Drugs and bioactive agents are encapsulated, embedded or incorporated within the bulk phase of the fibers, so that their release kinetics depends on their diffusion out of the fiber and the fiber degradation/erosion. For example, in Xie et al., (Xie et al., Electrospun micro- and nanofibers for sustained delivery of paclitaxel to treat C6 glioma in vitro, Pharm. Res. 23 (2006), 1817-1826) a biodegradable polymer solution containing hydrophobic anti-cancer drugs such as paclitaxel was directly electrospun to produce drug releasing nanofibrous mesh. Also, Xu et al. (Xu et al., BCNU-loaded PEG-PLLA ultrafine fibers and their in vitro antitumor activity against Glioma C6 cells, Journal of Controlled Release 114 (2006) 307-316) developed implantable BCNU-loaded polymer fibers for the controlled release of BCNU. This antineoplasic agent was well incorporated and dispersed uniformly in biodegradable poly(ethylene glycol)-poly(lactic acid) (PEG-PLLA) copolymers nanofibers by using the electrospinning method. In patent application WO 2009/064767 an antimicrobial nanofiber is formed from an electroprocessed blend of cellulose acetate as a polymer material, chlorhexidine (CHX) as an antimicrobial agent and an organic titanate as a crosslinker in such a way that CHX was covalently linked to the nanofiber. Patent application WO 2009/133059 discloses nanofiber matrices formed by electrospinning a solution of a hyperbranched polyester and creatine monohydrate for the controlled release of creatine.
However, many interesting bioactive agents are protein or nucleic acid in nature that do not dissolve in organic solvent and may suffer loss of bioactivity when dispersed in the polymer solution. Co-axial electrospinning, where the drug is dissolved in an aqueous core solution and the polymer in an organic shell solution, is one approach to overcome this drawback by extruding the core and shell solutions individually through two concentric nozzles. In patent application WO 2008/013713 coaxial electrospun nanofibers are disclosed having a core and a polymeric shell surrounding the core, wherein a growth factor or an adenovirus is encapsulated within the core.
In some cases, surfaces of electrospun nanofibers can be chemically functionalized for achieving sustained delivery through physical adsorption of diverse bioactive molecules such as proteins, enzymes, growth factors or drugs. For example, therapeutic proteins and nucleic acid were physically immobilized for controlled delivery (Patel et al., Bioactive nanofibers: synergistic effects of nanotopography and chemical signaling on cell guidance, Nano Lett. 7 (2007) 2122-8), and antibacterial agents were physically immobilized for immediate release from the nanofiber surface (Bolgen et al., In vivo performance of antibiotic embedded electrospun PCL membranes for prevention of abdominal adhesions, J. Biomed. Mater. Res. B Appl. Biomater. 81B(2007) 530-543).
In recent times, Cai et al. (Cai et al., International Journal of Pharmaceutics 419 (2011) 240-246) have proposed the sustained release of 5-fluorouracil (5-FU) by incorporating it into sodium carboxymethylcellulose sub-micron fibers prepared by freeze-drying as an alternative of electrospun nanofibers. The drug release from this swellable matrix is mainly based on diffusion out of the fibers. A similar system was proposed in patent application US 2005/0158362A1, in which bioactive compounds such as bovine serum albumin are suspended or solved in the polymer solution prior to electrospinning, resulting in a system with active agent mainly loaded within the nanofibers.
Because release from active agent-loaded polymer fibers is highly dependent on the composition of the fibers, the active agent-to-polymer ratio, the co-loading of other substances and the thickness of the fibers, newer approaches have been developed that load the active agent in polymer microspheres that control their release from the fiber mesh. Patent WO 2010/096254 developed formulations in which bovine serum albumin (BSA) or chondroitin sulfate are loaded in polystyrene (PS or PLGA) microspheres and then loaded in nanofibers made of electrospun polycaprolactone (PCL) and poly(ethylene oxide) (PEO). The PS microspheres can reside within one fiber or adjacent to a first type-fiber, a second type-fiber or both. The release kinetics of BSA depends upon its diffusion out the corresponding fiber, the degradation of this fiber as well as the degradation of the polymeric microsphere shell.
Recently, Wang et al. (Wang et al., Fabrication and Characterization of Prosurvival Growth Factor Releasing, Anisotropic Scaffolds for Enhanced Mesenchymal Stem Cell Survival/Growth and Orientation, Biomacromolecules 2009, 10, 2609-2618) have developed nanofiber scaffolds for tissue engineering that release a insulin growth factor (IGF-1) to induce cellular growth and survival. Such scaffolds are formed by electrospinning polyurethanurea nanofibers and IGF-1-loaded microspheres assembled into the scaffold. Encapsulation of growth factors protects them from proteolysis and allows their sustained release; the release kinetics are dependent upon polymer concentration, molecular weight and growth factor loading in microspheres.
However, the process of manufacturing nanofibers loaded with polymer microspheres containing active agents is challenging because of the complex technical processes required for microsphere production, isolation, sterilization and loading within the nanofiber mesh. In addition, the encapsulation efficiency of the active agent into the microspheres is usually suboptimal. The stability of the active principle can also be affected by solvents used during the microencapsulation process. So there continues to be a need in the state of the art to provide alternative local drug delivery systems for the sustained and controlled delivery of therapeutic agents.
The present inventors have discovered that when active principles are formulated as microparticles of the pure active principle and are entangled between the fiber mesh of a nonwoven membrane of biocompatible electrospun nanofibers, they can be locally released in a sustained way. That is, the microparticles prepared are suspended in a nonsolvent and poured into the nanofiber mesh. As a result, they are physically retained in the membrane between the nanofibers and cannot be released to the external medium but in their solubilised form. Such approach is especially appropriate for active principles of limited water solubility. Release occurs when the physiological fluids fill the membrane and solubilize the microparticles once located in the body area to be treated. Even though the active agent particles are not protected by a polymeric shell like the drug-loaded microsphere of the state of the art, they can be released in a sustained, efficient and oriented manner.